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  • Title: Effects of substance P on [Ca2+]i and force in intact guinea pig ileal smooth muscle.
    Author: Matthijs G, Himpens B, Peeters TL, Vantrappen G.
    Journal: Am J Physiol; 1990 Jul; 259(1 Pt 1):C150-60. PubMed ID: 1695485.
    Abstract:
    In intact sheets of the guinea pig ileal longitudinal muscle, loaded with fura-2, both substance P (SP)- and K(+)-induced contractile responses are preceded by cytoplasmic free Ca2+ ([Ca2+]i) variations. In response to SP (10(-10)-10(-7) M), [Ca2+]i and force increased concentration dependently. From 10(-9) M on, the response was biphasic: an initial Ca2+ spike and force transient were followed by a tonic component. The [Ca2+]i and force vs. log [SP] curves were sigmoidal for the initial phasic component, while a homologous receptor desensitization caused a reduced tonic component of the [Ca2+]i and contractile response at higher concentrations of SP. Both intracellular Ca2+ release and Ca2+ influx play a role in the effect of the peptide. In depolarizing solutions (140 mM K+) and in Ca2(+)-free medium (2 mM EGTA), SP induced a transient increase in [Ca2+]i and force. The Ca2+ stores used by SP and acetylcholine (ACh) overlap. [Ca2+]i and force fell to base-line levels when the extracellular Ca2+ was reduced from 1.2 to 0.2 mM during stimulation with SP. Verapamil reduced the tonic response. We also studied the relation between [Ca2+]i and force for the peak and steady-state values after stimulation with increasing concentrations of SP and K+. The phasic force response was linearly related to log [Ca2+]i. During the sustained response to K+, the Ca2+ sensitivity of the contractile/regulatory proteins was decreased, whereas no changes were observed during prolonged stimulation with the peptide.
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